The invention is directed to optics and more particularly to optics through which spaced apart laser light beams emerge from a non-linear frequency crystal can be coaxially converged to produce a high efficiency higher harmonic laser light beam.
Due to the optical anisotropic nature of frequency doubling crystals, the propagation of a laser beam in such a crystal usually takes different directions for its phase front (wave vector) and its energy flow (ray vector). In a second harmonic generation process, the wave vectors of both the fundamental and harmonic beams are the same, owing to a condition called phase matching. However the ray vectors of the two beams are usually different. This is called the walk-off effect. A typical walk-off angle between the two beams is about one degree in commonly used frequency doubling crystals. As a result, at the output of the crystal, the fundamental and the second harmonic beams are parallel with a small transverse displacement. If the two beams are then directed onto a crystal for sum frequency generation, this transverse displacement will effect the efficiency of the third harmonic generation.
A present art method of producing third harmonic beams is to separate the fundamental and second harmonic beams and bring them back together to a near coaxially overlapped condition by directing the two separated beams into a dichroic mirror and then a recombination through a plurality of optics by transverse repositioning. This method requires a plurality of optic elements each of which require extensive time consuming precise placement manipulation. Additionally, the dichroic mirrors must satisfy the characteristics that for allowing high transmission efficiency at one laser wavelength and high reflection at the second wavelength. This is technically difficult to achieve, resulting in loss of laser intensity of either or both of the laser beams. Even when the plurality of optic elements are positioned as precise as possible, considerable third harmonic beam power is lost.
The present method of developing third harmonic beams has a maximum efficiency of only about 30 percent.
There has not been a simple yet precise development of third harmonic laser beams having increased maximum efficiency until the emergence of the instant invention.